A cathode ray tube (CRT) has been a dominant display technology since the introduction of television. In operation, the CRT relies upon cathodoluminescence to emit light which forms the desired pictures. More precisely, an emissive material, commonly phosphor, is bombarded with a beam of high energy electrons to cause the phosphor to emit light to form a desired image. The intensity of the emitted visible light coming from the excited phosphor is uniform in all directions which provides the CRT with excellent viewing angles. In other words, one need not be located directly in front of the CRT in order to view the images on the CRT screen. The ubiquitous CRT is more than adequate in terms of resolution, cost, and brightness for screen diagonal lengths ranging from 5 inches to about 45 inches. Below and above these lengths, however, the depth, weight, and power consumption of the CRT become prohibitive for most display applications.
Another display technology, namely the flat panel display, has steadily improved in resolution, cost, and brightness over recent years. The flat panel display is thin, light weight, and, in general, does not require as much power during normal operation as the CRT. In fact, the flat panel display is increasingly displacing the CRT in a broad range of display applications.
One of the leading technologies for the flat panel display is the liquid crystal display. With reference to FIG. 1, a common liquid crystal display 10 has two white-light fluorescent tubes 1 located at the top and bottom of the display 10. The white light emitted by the fluorescent tubes 1 is directed through a light pipe 2 which guides the white light from the top to the bottom of the display 10 while allowing some of the light to escape in the direction of a diffuser 3. The diffuser 3 uniformly distributes the white light so as to minimize variations in brightness across the surface of the display 10.
The diffused white light is polarized by a rear polarizer 5 which ensures that the white light has a preferred direction of polarization. The rear polarizer 5 is adhered to a rigid, or at least semi-rigid, transparent substrate 7 which is usually made of soda-lime or borosilicate glass, but may be made from other materials, such as quartz or plastic. An optically transparent and electrically conductive electrode pattern 9 is deposited and patterned on the transparent substrate 7 to form the row electrodes of a passive-matrix.
A first alignment layer 11 is deposited on the electrode pattern 9 and has grooves formed in a first direction by a rubbing treatment. A nematic liquid crystal layer 13 is located between the first alignment layer 11 and a second alignment layer 15. The nematic liquid crystal 13 forms a director by aligning with the grooves created in the first and second alignment layers 11 and 15. Alignment layer 15 has grooves formed perpendicular to the grooves in the first alignment layer 11 whereby the director for the nematic liquid crystal 13 twists 90 degrees. Some liquid crystal displays have directors with as much as 270 degrees of twist from alignment layer 11. The alignment layers 11 and 15 are generally composed of a polyamide or silicon dioxide.
The alignment layer 15 is deposited on top of a second electrode pattern 17 which forms the column electrodes of a passive-matrix. The electrode pattern 17, as well as electrode pattern 9, is comprised of an optically transparent and electrically conductive material, such as indium tin oxide (ITO). A passivation layer is preferably provided between the electrode pattern 9 and alignment layer 11 and also between the electrode pattern 17 and alignment layer 15. The passivation layers insulate the electrode patterns 9 and 17 from each other and are commonly comprised of silicon monoxide or silicon dioxide.
The liquid crystal display 10 is a color display and accordingly has a color filter mosaic pattern 19, upon which the electrode pattern 17 is deposited. The color filter mosaic pattern 19 has three color filter materials, an opaque material, and an optional planarizing and transparent acrylic topcoat. The color filters, typically made from molecular dyes or pigments, transmit only one of the primary colors, namely red, green or blue. The color filters absorb all other regions of the visible spectrum present in the white light emitted by the fluorescent tubes 1 and are patterned such that each pixel is completely covered by the color filters. The opaque material surrounds the individual color filter dots and forms a black matrix around them which improves contrast. A transparent topcoat, typically made from an acrylic material, may be deposited on top of the color filter mosaic pattern 19 to form a planar surface which improves deposition of the electrode pattern 17.
The color filter mosaic pattern 19, the electrode pattern 17, and the alignment layer 15 are sequentially deposited on top of a front transparent substrate 21. The front transparent substrate 21, as with the transparent substrate 7, may be rigid or at least semi-rigid, is typically comprised of soda-lime float glass, but may be formed from glass, plastic, or quartz.
A front polarizer 23, commonly referred to as an analyzer, is adhered to the front transparent substrate 21. The analyzer 23 absorbs a certain amount of light coming out of the liquid crystal cell 27, which includes all elements from the rear polarizer 5 to the analyzer 23. More specifically, the analyzer 23 absorbs an amount of light which is proportional to the degree of shift in the light's direction of polarization from the rear polarizer 5. The shift in the direction of polarization is caused by electric and magnetic fields which are produced by voltages applied to the electrode patterns 9 and 17 and which change the orientation of the director formed in the nematic liquid crystal layer 13. Finally, optional anti-reflection treatments 25, in the form of multi-layer optical thin film stacks, may be applied to analyzer 23 to reduce the intensity of specular reflections from ambient light.
A problem with liquid crystal displays in general, however, is that the displays do not efficiently transmit light. Each of the layers forming the liquid crystal display 10 is associated with a certain amount of loss in transmitted light. For example, approximately 67% of the visible energy from the backlight 1 is absorbed by just the color filters 19. It is therefore a problem in the industry to improve the efficiency of a liquid crystal display.
Another problem with many liquid crystal displays is that they offer a very limited viewing angle. In order to satisfactorily view an image on the liquid crystal display 10, the viewer must be positioned at an angle fairly close to directly in front of the display 10. It is thus a desire in the industry to produce images on a liquid crystal display which can be viewed from a wider viewing angle.
The cost of manufacturing a liquid crystal display is relatively high. One of the largest costs involved in manufacturing a liquid crystal display 10 is the color filter mosaic pattern 19. The color filter mosaic pattern 19 must be chromatically accurate if the display 10 is to produce accurate images. The color filter mosaic pattern 19 achieves this high degree of precision by employing very expensive organic dyes which must be uniformly deposited and light stable. In addition to the various layers shown in FIG. 1, many liquid crystal displays are also fabricated with retardation films or double domains to counteract the effects of birefringence. Also, active-matrix liquid crystal displays often have a significant number of defective transistors. The color filter mosaic pattern, the retardation films, double domains, and low yields from defective transistors cause the price of the liquid crystal display 10 to be relatively high.
Another problem associated with liquid crystal displays is flicker. A liquid crystal display with an active-matrix pattern includes an integrated storage capacitor per pixel for holding a necessary electric field throughout the frame time. The storage capacitors in active-matrix liquid crystal displays therefore hold the image on the display between successive images, thereby preventing flicker. The liquid crystal displays with a passive-matrix pattern, on the other hand, do not have any storage capacitor but instead rely upon the intrinsic capacitance of the liquid crystal material to reduce flicker between successive images. The passive-matrix liquid crystal displays therefore employ slow response liquid crystal materials which eliminate the flicker but, due to their slow response, prevent the displays from having full-motion video capabilities. The slow response liquid crystal materials also cause smear during the display of fast-motion images. It has therefore been a problem in the display industry to provide a passive-matrix liquid crystal display which has neither flicker nor smear and which provides full-motion video capability.
In order to improve the transmittance efficiency, liquid crystal displays have been designed with phosphor layers. For instance, U.S. Pat. No. 5,146,355 to Prince et al. has a phosphor layer for receiving ultraviolet light and for converting the ultraviolet light into visible light. Also, Kevin Walsh et al. proposed a liquid crystal display having phosphors in "Improved lighting efficiency for active-matrix liquid crystal displays" in SPIE Vol. 2219 Cockpit Displays, 1994. In general, with these liquid crystal displays, the phosphors are located within the liquid crystal cell itself and generate visible light, which is subsequently modulated by the liquid crystal cell.
While the structure in these displays may improve the transmittance efficiency, the liquid crystal displays are still prone to many of the above problems. For instance, the liquid crystal displays would still have problems with birefringence and associated limited viewing angles, flicker, and smear. The response of the liquid crystal displays is still limited by the characteristics of the liquid crystal material. Thus, the displays with a passive matrix would need to employ a slow response liquid crystal material in order to reduce flicker, but would still be prone to smear and be unable to display full-motion video.
Additionally, the liquid crystal displays would have a low contrast. The phosphor materials positioned within the liquid crystal cell have a lambertian emission characteristic whereby light is emitted within an angle of about 180 degrees. Because the light is highly divergent, light from a single phosphor will cross-over into other pixels, thereby reducing the contrast of the liquid crystal display. Thus, the benefit of a higher transmittance efficiency will be offset by the lower contrast.
A further difficulty with the liquid crystal displays in Prince et al. and Walsh et al. is that the displays will be fairly expensive to manufacture. Both of these displays will require a change in the typical method in which a liquid crystal cell is manufactured so that the phosphor layers, as well as planarizing layers to offset the irregularly shaped surfaces of the phosphor layers, can be included within the cell. In addition to the phosphor layers and planarizing layer, the cells will also require other layers, such as dichroic filters and polarizers, all of which substantially increase the cost of the liquid crystal cell. These additional layers also increase the distance that the light diverges before reaching the liquid crystal material and, consequently, increases cross-talk and lowers the contrast of the displays. To improve the contrast, the displays would still require the expensive color filters inside the cell, which further increases the cost of the displays. These additional layers within the liquid crystal cell will most likely require thin film manufacturing techniques which will also raise the cost of the displays and lower the manufacturing yields. Therefore, the liquid crystal displays in Walsh et al. and Prince et al. will be significantly more expensive to manufacture than a comparable conventional display.
A need therefore exists for a liquid crystal display which has a higher transmittance efficiency, which is free from birefringence, which does not exhibit flicker between successive images, which does not exhibit smear, and which has an optimal viewing angle. A need also exists for a liquid crystal display which has a higher contrast and which is more inexpensively manufactured.